In the steady observation, that is, measurement of sea waves, an under-water pressure type wave meter has been generally used for measuring pressure fluctuations at the seabed.
The under-water pressure type wave meter can be used in shallow waters but has a limited use in very deep waters with a lower sensitivity to short-period waves since the movement of water particles due to sea surface waves does not reach the seabed.
Thus, when the under-water pressure type wave meter is used, observed waveforms are obtained through a low-pass filter, inevitably reducing the accuracy and reliability of observation data.
Meanwhile, an ultrasonic wave meter can directly obtain waveforms on the sea surface as compared to the under-water pressure type wave meter, and has been widely used on the coasts all over the country.
The ultrasonic wave meter observes sea surface waves by transmitting sharp ultrasonic pulse beams toward the sea surface from a transmitter/receiver on the seabed, receiving reflected waves from the sea surface, and continuously recording the round-trip propagation time (corresponding to the sea surface level) of the ultrasonic pulse (see, for example, “Enganharo Kaisho Kansoku Deta no Kaiseki Katsuyo nikansuru Kaisetsusho (Manual for Analysis and Utilization of Coastal Waves/Oceanographic Observatory Data) (pages 3 to 4), published by Coastal Development Institute of Technology, in August of 2000”).